This invention relates to two-stroke cycle engines adapted for use of bush cutters, hedge trimmers, or the like, and more particularly, to a two stroke cycle engine which can achieve an improvement of combustion efficiency as well as reduction in total hydrocarbons in exhaust gas.
Such a two stroke cycle engine has been designed such that all the charging, scavenging and exhausting permit replacement of gases with high efficiency to provide a high power output. However, due to the fact that the scavenging and exhausting occur simultaneously, fuel mixture introduced through scavenging ports into a combustion chamber escapes into the exhaust port and this causes an increase in total hydrocarbons in the exhaust gas. In addition, after the fuel mixture has been burned in the combustion chamber, the burned gases are exhausted through the exhaust port with some unburned gas contained therein and this also causes an increase in total hydrocarbons in the exhaust gases. To reduce the total hydrocarbons in the exhaust gases, measures may, therefore, be adopted for preventing the fresh fuel mixture from escaping into the exhausting port and for increasing the combustion efficiency. Due to increase in combustion efficiency, total hydrocarbons in the exhaust gas can be reduced while improving the power output.
For purposes of preventing escape of the fuel mixture, scavenging flow of the gases has been improved by changes in positions and configurations of the scavenging and exhaust ports, and a shape of the combustion chamber has been modified to increase the combustion efficiency, but such improvements of the scavenging flow and the combustion efficiency have their limitations. It has also been proposed hitherto to make smaller an angle of rotation of the crankshaft through which the exhaust port opens, in order to decrease an amount of escaping fresh fuel mixture and slightly increase a residue of the burned gases in the combustion chamber to facilitate burning the fresh fuel mixture, thereby increasing the combustion efficiency. However, this results in a decrease in power output due to the exhaust efficiency being decreased. Since both the facts that the power output is increased due to the highly efficient replacement of gases and that the total hydrocarbons are decreased due to the prevention of escape of fresh fuel mixture into the exhaust port are inconsistent with each other, the improvement of the power output of the engine and the reduction in total hydrocarbons in the exhaust gases must be achieved in a balanced relation.